High-frequency probes are required for the testing of fine layers as well as for the ultrasonic detection of minor faults. If the fault to be tested or the layer thickness to be checked becomes small compared to the wave length of the ultrasonic signals used, the non-homogeneity no longer represents an obstacle to the ultrasonic signal. The invention relates to probes having a frequency that is high enough to detect fine layers as well as minor faults. The probes typically have a frequency bigger than 20 MHz, the frequency generally ranges between 10 and 100 MHz, it may also be of 150 MHz and more.
The article of J. Buchler, M. Platte and H. Schmidt "Electronic circuit for high-frequency and broad band ultrasonic pulse-echo operation" in Ultrasonics, 1987, Vol 25, March, S. 1112 through 1114 discloses a probe with a transceiver transducer made of a thin PVDF-foil (polyvinylidenefluoride). Said foil is slowly charged by a transmitter electronics until a transmitting potential is attained and then, it is discharged as fast as possible. The discharge occurs by short-circuiting the transducer by means of an appropriate switch, which is embodied in the article by a transistor switch. The time of discharge is indicated as of 10 ns for a potential jump of 150 V, the capacity of the transducer designed as a foil is of 300 pF.
The ultrasonic pulse achieved is essentially unipolar and very short, his width is of approximately 50 ns.
The disadvantage of the probe of the art is that after an ultrasonic pulse has been sent out, a quite long period of waiting time elapses until it is ready for reception. The periods of idle time which have to be observed are not always acceptable for practical testing. Although it is on principle possible to work with a sufficiently long pre-located body and thus avoid the disadvantages of the periods of idle time that are to be observed, this has the disadvantage that, with the high-frequency pulses under discussion, the sound in the pre-located body is considerably attenuated. This disadvantage should be avoided.
The ultrasonic testing devices working according to the pulse-echo method usually are provided with a probe with a transceiver transducer as it is known out of the above-mentioned article. In principle, probes with separate transducers, that is with a transmitter transducer and a receiver transducer are also known. As an example, we are referring to the DE-book J. Krautkramer and W. Krautkramer "Materials Testing with Ultrasound" published by Springer, 6.sup.th ed. In the probes with two transducers of the art the transducers are arranged side by side in the direction of sound propagation. Thus, the path followed by the ultrasound between the transmitter transducer and the receiver transducer is V-shaped. But this is detrimental to the testing of layers, since the measurement should occur perpendicularly to the boundary surfaces of the layers.
That is where the invention comes to bear. The object of the invention is to provide a probe that may be used in particular in the high-frequency range and that allows of the reduction of idle time and thus of a shorter design of the pre-located body while keeping possibly unchanged the advantages of the probe according to the article mentioned.
Starting with the probe of the type mentioned above, the solution of this object is to have the transmitter transducer and the receiver transducer made of the same material and to have the transmitter transducer and the receiver transducer arranged one behind the other in the direction of sound propagation and connected in a plane, in particular to have them glued one on top of the other.
This probe is provided--as actually known--with separate transducers for sending out and receiving the ultrasonic pulse. But now, the transducers are arranged directly one behind the other. Thus, the probe according to the invention essentially works like a probe with one transducer, since the two transducers are arranged one behind the other and are made of the same material.
When a transmitter transducer and a receiver transducer are arranged one behind the other in the direction of sound propagation, reflections occur between the two transducers. According to the invention, these reflections are avoided by connecting the two transducers in a plane, in particular by gluing them one on top of the other. Attention is particularly payed to the fact that the space between the two transducers is as small as possible. By using two transducers made of the same material, no reflections occur on the boundary surfaces.
The invention suggests therefore a spatial design of the two transducers being as similar as possible to the design of a probe with one transducer. The two transducers are electrically separated from each other though, so that a possible interference in the receiver electronics by the transmit pulse is largely excluded. Thus, the idle time is largely suppressed and is virtually no longer relevant. So far, it is possible to switch into receive position shortly after having sent out an ultrasonic pulse and, accordingly, the pre-travel to be used can be short. It is not always desirable to completely do without pre-travel, since the pre-travel also protects the transducers.
In a particularly preferred embodiment, the thickness of the glue layer between transmitter transducer and receiver transducer is as small as possible. It should be so small that there are virtually no reflections. The material preferably used for the glue layer is a material that has as far as possible the same sound characteristics as the material of which the transducers are made.